1. Field of the Invention
The present teaching relates to a lithium ion secondary battery. Incidentally, the present application claims priority based on Japanese Patent Application No. 2015-191845 filed on Sep. 29, 2015, and Japanese Patent Application No. 2016-111391 filed on Jun. 3, 2016, and the entire contents of the applications are incorporated herein by reference in the present specification.
2. Description of the Related Art
In recent years, lithium ion secondary batteries are light in weight, and can provide a high energy density, and hence have been growing in importance as on-vehicle power sources, or power sources for personal computers and portable terminals. Particularly, lithium ion secondary batteries have found a wider range of use as power sources for driving motors to be mounted on vehicles (which will be hereinafter referred to as vehicle driving power sources).
Incidentally, so-called high-rate charging/discharging in which charging and discharging are performed with a large current for a short time is performed with a lithium ion secondary battery to be mounted on a vehicle such as a car, and to be used as a vehicle driving power source. During the period in which such high-rate charging/discharging is carried out, in the electrodes (the positive electrode and the negative electrode) of the lithium ion secondary battery, sharp insertion or desorption of lithium ions of electric charge carriers, and the like result in a change in structure of the electrode active material. This may cause expansion or shrinkage (expansion and shrinkage will be hereinafter collectively referred to as “expansion/shrinkage”) of the electrode active material layer. Such expansion/shrinkage of the electrode active material layer may cause the non-aqueous electrolyte included in the electrode to leak out of the electrode (particularly, the electrode active material layer).
Particularly, with a lithium ion secondary battery including a graphite type carbon material as a negative electrode active material, the degree of expansion/shrinkage of the negative electrode active material layer during high-rate charging/discharging is large. Thus, the non-aqueous electrolyte tends to readily leak out. Such leakage of the non-aqueous electrolyte from the negative electrode means that the lithium salt included in the leaking electrolyte also leaks together. This incurs a fear of the reduction of the lithium salt concentration in the negative electrode active material layer.
Further, the leakage of the electrolyte and the reduction of the lithium salt concentration entailed by expansion/shrinkage of the negative electrode active material layer cause a lithium salt concentration variation at the surface and inside the negative electrode (negative electrode active material layer). When such a lithium salt concentration variation (particularly, the concentration variation in the plane direction of the negative electrode) is caused, large-resistance points are formed spotwise in the negative electrode active material layer. This is not desirable in that the degradation of the battery performances, such as the degradation of the cycle characteristic (durability) and an increase in internal resistance, is caused.
In this regard, for example, Japanese Patent Application Laid-open No. 2002-231316 discloses a lithium ion secondary battery characterized in that a low-crystallinity carbon material having a lattice spacing (d002) of 0.372 nm to 0.400 nm is used as the negative electrode active material. Then, Japanese Patent Application Laid-open No. 2002-231316 describes the following: the low-crystallinity carbon material originally has a crystal structure with a lattice spacing (d002) of 0.372 nm or more when lithium is inserted into graphite; accordingly, the negative electrode active material (the low-crystallinity carbon material) does not repeat expansion/shrinkage even during charging/discharging of the battery, resulting in a favorable cycle characteristic.